Rotary valves embodied as axial or radial rotary valves are used to control the steam throughput in a steam turbine. With the immovable fixed ring and the rotary ring which is arranged concentrically and rotatably on said fixed ring, the angular position of which can be changed by a servomotor, it is possible to open or close the control slots provided in the two rings to a greater or lesser extent. Compared with the valves used otherwise in steam turbine construction for controlling steam removal, rotary valves of this type are embodied relatively simply and still also feature a few other advantages.
A generic rotary valve for a steam turbine is known from DE 19 620 949 A1. The known rotary valve only achieves the best degree of efficiency for the steam turbine when fully open, since in this position the control slots are optimally passed through. If the rotary valve moves in the closing direction, by the profile heads in the rotary ring being moved upstream of the control slots in the fixed ring, strong turbulences result in the partially closed state of the rotary valve on the edges of the profile heads and the profile ends. As a result of such turbulences, the degree of efficiency of the steam turbine is significantly worse in the case of a partial load.